1. Field of the Invention
The present invention relates to a liquid crystal display device. More particularly, it relates to an in-plane switching mode liquid crystal display (IPS-LCD) device and a fabricating method thereof.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices are being developed as the next generation of display devices because of their characteristics of light weight, thin profile, and low power consumption. In general, an LCD device is a non-emissive display device that displays images by making use of a refractive index difference through utilizing optical anisotropy properties of a liquid crystal material interposed between an array substrate and a color filter substrate. Of the different types of known liquid crystal displays (LCDs), active matrix LCDs (AM-LCDs), which have thin film transistors (TFTs) and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superiority in displaying moving images.
FIG. 1 is a perspective view of a liquid crystal display device according to the related art. As shown in FIG. 1, an upper substrate 10 and a lower substrate 30 are spaced apart from and face each other, and a liquid crystal layer 50 is interposed therebetween. A plurality of gate lines 32 are formed on the inner surface of the lower substrate 32 and a plurality of data lines 34 cross the plurality of gate lines 32. A thin film transistor (TFT) “T” is connected to the gate line 32 and the data line 34. A pixel region “P” is defined by the crossing gate line 32, data line 34 and pixel electrode 46. Although not shown in FIG. 1, the TFT “T” includes a gate electrode that receives a gate voltage is applied, source and drain electrodes for passing a data voltage through the pixel electrode 46, and a channel region that is controlled by the gate voltage.
A color filter layer 12 and a common electrode 16 are sequentially formed on inner surface of the upper substrate 10. The color filter layer 12 transmits light only having a specific wavelength band. Although not shown in FIG. 1, a black matrix is formed on the color filter layer 12. The black matrix prevents light from passing through that is not in the specific wavelength band.
An upper polarizing plate 52 and a lower polarizing plate 54 are disposed outside the upper substrate 10 and the lower substrate 30, respectively. The upper polarizing plate 52 and the lower polarizing plate 54 transmit light that only has an optical axis parallel to the polarization axis of the respective polarizing plate. A backlight unit can be disposed below the lower polarizing plate 54, as shown in FIG. 1.
The liquid crystal display (LCD) device is fabricated through a liquid crystal cell process. In the liquid crystal cell process, a liquid crystal layer is formed between an array substrate and a color filter substrate. The array substrate has a switching element and a pixel electrode while the color filter substrate has a color filter layer and a common electrode. As compared with processes for the array substrate and the color filter substrate, a fabrication step for the forming the array substrate is seldom used to form the color filter substrate and vice versa. A liquid crystal panel, which is a basic element of an LCD device, is completed through a liquid crystal cell process that brings together the array substrate and the color filter substrate. The liquid crystal cell process may be divided into an orientation treatment step for providing orientation of the liquid crystal layer, a cell gap formation step, a cell-cutting step and a liquid crystal injection step.
FIG. 2 is a plane view showing an orientation treatment process for a twisted nematic mode liquid crystal display device according to the related art. As shown in FIG. 2, a mother glass 60 includes a cell region “II” where one of array elements and color filters is formed. Since a rubbing direction determines a main viewing angle in a twisted nematic (TN) mode LCD device, the upper substrate and the lower substrate are generally rubbed along 45° and 135° directions “R1” and “R2, ” respectively, which cross each other along the diagonal direction of an LCD panel. In other words, since a rubbing process is performed along the diagonal direction of the mother glass 60, a rubbing roll 62 having a length “L2” corresponding to the diagonal length “L1” of the mother glass 60 is necessary. Especially for a large area mother glass, however, there are many problems in obtaining and using a rubbing roll having a length corresponding to the diagonal length of the mother glass.
In a TN mode LCD device, since a rubbing direction determines a viewing angle property of the LCD device, the rubbing direction is fixed to a specific direction. As a result, the rubbing direction may not be freely selected. In addition, since the rubbing process is performed along a diagonal direction of a mother glass, the cost of the rubbing process increases for a large area mother glass due to the need for a long rubbing roll and corresponding apparatus for a long rubbing roll.
In a conventional LCD device, since the pixel electrodes and common electrodes are positioned on the lower and upper substrates, respectively, a longitudinal electric field is induced perpendicularly between the lower and upper substrates. The conventional LCD devices have high transmittance and high aperture ratio. However, the conventional LCD devices using the longitudinal electric field have a drawback in that they have a very narrow viewing angle. In order to solve the problem of the narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed.
The IPS-LCD devices typically include a lower substrate on which pixel electrodes and common electrodes are disposed. A liquid crystal layer is interposed between the upper and lower substrates. The upper substrate does not have any electrodes. A detailed explanation about the operational modes of a typical IPS-LCD panel will be provided while referring to FIG. 3.
FIG. 3 is a cross-sectional view of an IPS-LCD device according to the related art. As shown in FIG. 3, an upper substrate 80 and a lower substrate 70 are spaced apart from each other, and a liquid crystal layer 90 is interposed therebetween. The upper substrate 80 and the lower substrate 70 are often referred to as a color filter substrate and an array substrate, respectively. A common electrode 72 and a pixel electrode 74 are formed on the lower substrate 70. The common electrode 72 and pixel electrode 74 are positioned such that they are parallel to each other. On the surface of the upper substrate 80, a color filter layer (not shown) is commonly positioned to correspond to an area between the pixel electrode 74 and the common electrode 72 of the lower substrate 10.
A voltage applied across the common electrode 72 and the pixel electrode 74 produces an in-plane electric field “IF” through the liquid crystal molecules 92 of the a liquid crystal layer 90. The liquid crystal molecules 92 have a positive dielectric anisotropy, and thus it aligns parallel to the electric field “IF.” In other words, when a voltage is applied across the common electrode 72 and the pixel electrode 74, i.e., “on state”, a lateral electric field “IF,” which is parallel to the surface of the lower substrate 70, forms between the common electrode 72 and the pixel electrode 74 on the lower substrate 70. Accordingly, the LC molecules 92 are arranged such that their longitudinal axes into coincidentally aligned with the electric field “IF.” Since the LC molecules switch directions while maintaining their longitudinal axes in a plane perpendicular to the direct viewing direction of a display, in-plane switching provides a wide viewing angle for a display device. The viewing angles can range 80 to 85 degrees in up-and-down and left-and-right sides from a line vertical to the IPS-LCD panel, for example.
FIG. 4A is a plane view of an array substrate according to the related art IPS-LCD device, and FIG. 4B is a plane view of an array substrate according to another related art IPS-LCD device. The common electrode and the pixel electrode of FIG. 4A are in a stripe pattern, and the common electrode and the pixel electrode of FIG. 4B are in a zigzag pattern. As shown in FIGS. 4A and 4B, gate line “GL” is transversely arranged across the figures and data lines “DL” are disposed substantially perpendicular to the gate lines “GL.” A common line “CL” is also transversely arranged across the figure in parallel with the gate line “GL” and is spaced apart from the gate line “GL.” The gate line “GL,” the common line “CL” and the data line “DL” define a pixel region “P” on the array substrate. A thin film transistor (TFT) “T” is disposed adjacent to a corner of the pixel region “P” near the crossing of the gate and data lines “GL” and “DL.”
As shown in FIG. 4A, a plurality of common electrodes 94 extend from the common line “CL” and are parallel to the data line “DL.” A plurality of pixel electrodes 96 are connected to a thin film transistor “T” and are parallel to the data line “DL.” The plurality of pixel electrodes 96 alternate with the plurality of common electrode 94.
As shown in FIG. 4B, common electrodes 97 and pixel electrodes 98 are shaped in zigzag pattern to create multiple domains. FIG. 4A and FIG. 4B have similar features. Accordingly, some of detailed explanations with regard to FIG. 4B, especially previously explained with reference to FIG. 4A, will be omitted in order to prevent duplicate explanations.
In FIG. 4A and FIG. 4B, an area “AA” between the common electrodes 94 and 97 and the pixel electrodes 96 and 98 may be referred to as an aperture area. The liquid crystal molecules in the aperture area are re-arranged by an electric field. For convenience, the common electrodes 94 and 97 and the pixel electrodes 96 and 98 are represented by an in-plane electric field electrode “IFE.”
In FIG. 4A, a rubbing process is performed along a first rubbing direction “RD1” that forms a certain angle with the in-plane electric field electrode “IFE.” The reason for inclining rubbing direction with respect to the in-plane electric field electrode “IFE” is to obtain a fast re-arrangement of the liquid crystal molecules in correspondence with the electric field. For example, the first rubbing direction may have an angle of 60° to 85° with respect to the gate line “GL.”
As shown in FIG. 4B, a second rubbing direction “RD2” inherently has an inclination with respect to the in-plane field electrode “IFE” because the in-plane field electrode “IFE” has a zigzag shape having an inclined angle. For example, the second rubbing direction “RD2” should be parallel with the data line “DL.”
In the IPS-LCD device according to the related art, the rubbing direction is limited by the shape of electrodes generating an in-plane electric field. Accordingly, it is very difficult to reduce fabrication cost of a rubbing process by adopting optimum apparatus.